Referring to the drawings, a conventional method for producing a semiconductor device is explained. FIGS. 10 to 12 show cross-sectional views for illustrating the conventional method for producing a semiconductor device step-by-step. FIGS. 10 to 12 are fractionated for convenience of the drawing.
Referring to FIGS. 10 to 12, a thermally oxidized film 12 is formed on a silicon substrate 1 to a thickness of 10 nm (FIG. 10b), and a nitride film 13 is deposited thereon to a thickness of 120 nm (FIG. 10c). This nitride film 13 is patterned to a field pattern, using a known lithographic technique (FIG. 11d). The resulting assembly is thermally oxidized at 980.degree. C. to form a field oxide film 3 to a thickness not less than 400 nm (FIG. 11e).
The nitride film 13 then is removed and the oxide film 12 beneath the nitride film 13 also is removed. Then, boron 6 is implanted on the entire wafer surface at an acceleration energy of 100 kev at a doze of 1.times.10.sup.12 to form a channel stop layer 14 (FIG. 12g).
Here, the field oxide film is explained. For a semiconductor device formed on the semiconductor substrate, local oxidation of silicon (LOCOS) is routinely used for device isolation. However, as the device size becomes finer, the device region is reduced, such that bird's beak like defects (termed as "bird's beak" herein) at LOCOS ends tends to affect the device region to an increasing extent. Since the bird's beak at the LOCOS end can be reduced by reducing the LOCOS film thickness, the LOCOS is recently reduced in thickness and is formed to a thickness on the order of 300 nm.